Transmissions utilized with large trucks, such as over-the-road vehicles, include transmissions having between twelve and sixteen forward speed ratios. In many instances, the transmissions currently available are actually a pair of separate transmissions arranged in series to obtain the high number of forward speeds with less pairs of gears. This concept of two transmissions in series causes the ratio progression to repeat as the transmission is stepped up or down through the speed ratios. This makes a geometric progression of the speed ratios the most practical. The vehicles have also generally been designed to expect a top gear, or maximum forward ratio of 1:1, which requires the addition of content to the planetary manual design transmissions. For this reason, most of the commercial trucks employ countershaft type transmissions wherein a plurality of synchronizer or mechanical clutches are included to establish the required speed ratios.
As is well known with countershaft type transmissions, the input shaft of the transmission generally drives a head gear set which then provides input speed to a countershaft on which a plurality of ratio gears are positioned. The ratio gears mesh with gears on the transmission main shaft, which is generally connected with the transmissions output shaft. Synchronizers or mechanical clutches on one of the shafts, main shaft or countershaft, are engaged to establish a drive connection between the respective shaft and gear member thereby the speed ratio on the countershaft to the main shaft is completed.
Planetary-type manual transmissions generally include a splitter gear or splitter planetary set forward of the multi-speed planetary set. For the most part, the planetary set is generally set to provide anywhere from six to eight forward speed ratios and the splitter set doubles that number of speed ratios. However, it is required that the speed ratios of the multi-speed planetary gearset be arranged such that the splitter gear set can effectively provide intermediate ratios. For this reason, the splitter gear set is usually alternately operated in an underdrive ratio and a direct ratio when the multi-speed planetary portion of the transmission is put through its extended range of gearing.